The Splanchnic Circulation

Question 1

Splanchnic Circulation

Answer 1

• Splanchnic circulation refers to the vasculature which brings blood to and from the major abdominal organs including the liver, spleen, stomach, pancreas, large and small intestine.
• The Splanchnic Vasculature is the major reservoir of blood in the body.
• refers to all organs fed by celiac, superior mesenteric and inferior mesenteric arteries

Question 2

Answer 2

More than 60% of the total splanchnic blood volume can reside in venules making the splanchnic venous circulation the primary source for mobilizing blood during crisis (e.g., hemorrhage).

Question 3

Fluid Exchange in the Splanchnic Circulation

Answer 3

• Fluid exchange in the splanchnic system also is very high. Secretions by the digestive tract amount to 7-8 liters/day with 1-2 liters/day of H2O ingested. This volume is approximately 2 to 3 times the body’s plasma volume.
• Thus, changes in fluid exchange between the GI tract and the systemic circulation can dramatically alter blood plasma volume.

Question 4

Hepatic Circulation

Answer 4

•refers to blood flow to the liver directly from the aorta

Question 5

Mesenteric Circulation

Answer 5

•refers to intestinal blood flow

Question 6

Organization of Blood Flow to and From Splanchnic Organs

Answer 6

• For most splanchnic organs blood flows through a branch of one of the three major arteries from the aorta and then after passing through the organ collects into the portal vein to drain to the liver.
• The liver also receives blood from the hepatic arterial branch of the celiac artery.

Question 7

Lymphatic Flow from the Alimentary Canal and GI organs is normally about […], but can reach up to […].

Answer 7

Lymphatic Flow from the Alimentary Canal and GI organs is normally about 5 ml/min, but can reach up to 25 ml/min.

Question 8

Blood Flow in the Portal Vein

Answer 8

• Blood collected from all of the alimentary organs flows into the portal vein.
• Blood with high bicarbonate levels (alkaline tide) arising from the stomach mixes within the portal vein with the relatively acid blood generated in the pancreas. Since the amount of bicarbonate produced in the pancreas is matched to the amount of acid formed in the stomach, the portal blood is neutralized prior to reaching the liver.
• The flow of portal vein blood to the liver also is important for the metabolic clearance of ingested/absorbed substances such as drugs and toxins.

-Thus, the positioning of the liver at the entrance for blood into the systemic circulation is important for screening substances absorbed from the alimentary canal.

Question 9

Under resting conditions, […] of cardiac output flows through the splanchnic vasculature, increasing to up to […] after a meal.

Answer 9

Under resting conditions, 20% of cardiac output flows through the splanchnic vasculature, increasing to up to 40% after a meal.

•The increase in flow during absorption of a meal is not uniform throughout all alimentary tissues but is localized to the areas which are most active.

• Blood flow to salivary glands and pancreatic acini increases only in the presence of stimuli that elevate their secretion.
• Within the alimentary canal itself, flow is regulated both longitudinally along the canal, (Segmental control) as well as, between the different layers of a given segment; i.e., mucosa vs. muscle (Transmural control).

Question 10

Segmental Control

Answer 10

•Flow along the length of the intestinal tract is regulated by tissue activity providing local or Segmental Control. Thus, distension by chyme moving into the stomach initiates signals for increased secretion (HCl), increased motility and elevated blood flow to an activated region.

Question 11

Transmural Control

Answer 11

• Blood flow to the different layers of the intestinal wall (transmural) is highest in the regions where activity is greatest.
• Generally, in the intestine the highest flow rates in an activated segment are found in the mucosa followed by the submucosa then muscle layers.
• Lymphatics 1 ml/min to 25 ml/min (high fat meal)

Question 12

Post-prandial Hyperemia Local Regulation of Blood Flow

Answer 12

• local metabolic regulation of blood flow
• After a meal (Post-prandial) blood flow to the alimentary organs increases substantially. The level of increase is determined by the state of activation of a region.
• Distension of any area of the alimentary canal elicits activation of transport (secretory or absorptive), and thereby metabolism within that segment increases.
• The buildup of metabolic end products (e.g., CO2, lactate, and adenosine) is a primary factor eliciting local vasodilation increasing blood flow to replenish substrate (O2) supply and remove absorbed nutrients.

•This autoregulation allows elevated blood flow to specific local regions where activity is high, due to the action of these vasoactive metabolites in altering the contractile state of feed arterioles within the alimentary wall.

Question 13

Autonomic Control of Blood Flow

Answer 13

• Sympathetic stimulation directly decreases blood flow to the alimentary canal through vasoconstrictor nerves (alpha-adrenergic).
• Sympathetic stimulation increases resistance, decreases splanchnic blood volume and increases systemic blood volume, thereby, elevating systemic pressure.
• Small veins and venules also are innervated though to a lesser degree than arterioles.
• Because the venous side is innervated, sympathetic outflow elicits significant redistribution of blood out of the splanchnic circulation by both decreasing flow to the organs and decreasing the reservoir volume on the venous side.
• Conversely, buildup of metabolites near areas of active absorption or secretion will autoregulate flow to those specific areas most in need of perfusion.

-This feature points to the local vs. central regulation of all processes associated with digestion-absorption; even under conditions that elicit strong central signals, alimentary function is most closely regulated at the local level.

•Parasympathetic: There is no apparent direct parasympathetic innervation of blood vessels in the splanchnic circulation. However, parasympathetic nerves can cause significant changes in metabolic activity of the alimentary canal, which then increases blood flow to the active regions.

Question 14

Sympathetic Escape

Answer 14

•As shown in the figure, stimulation of sympathetic nerves elicits a decrease in blood flow to the mesenteric vasculature. However, the segment under study was active (filled with chyme) and the buildup of local metabolites gradually causes an increase in flow to this region. This recovery of flow is referred to as Sympathetic Escape. Because overall flow is reduced leading to this buildup of metabolites, once sympathetic vasoconstriction is relieved, flow increases dramatically until the vasoactive substances are cleared, at which time flow returns to baseline.

Question 15

Countercurrent Exchange in the Intestinal Villus - Anatomy

Answer 15

• Blood flow within the villus is arranged such that the inflow of blood runs parallel to the vessels that drain the villus. Specifically, the collecting venule is located centrally in the villus while the capillaries form a web under the submucosa.
• This arrangement is characteristic of counter-current exchange systems. Since oxygen is removed as the blood moves through the capillaries, the oxygen tension in capillaries at the tip is low. The relatively de-oxygenated blood moves from the villus tip towards the mesenteric vein (at the villus base) within a central venule.
• This arrangement sets up an oxygen gradient from the capillaries to the central venule; that is, counter current exchange leads to gradients for specific molecules (in this case oxygen) between the villus base and villus tip.
• A similar argument can be made for Na+ concentration. In this case, as Na+ is absorbed all along the surface epithelia from base to tip, it becomes more concentrated as it gets to the tip prior to removal directly to the mucosal vein.

Question 16

Countercurrent Exchange…a potential problem

Answer 16

•Because the arterial and venous vessels are very close (20 microm in the villus shaft), lipid -soluble oxygen can short-circuit its usual vascular route by escaping the arteriole and passing directly into the venule. Therefore, some oxygen does not reach the capillaries, and thereby cells, at the tip of the villus. In pathological states associated with a severe reduction in mesenteric blood flow, the velocity of blood flowing through the villus also is reduced, thereby exaggerating the oxygen exchanger. Under such conditions, Ischemic hypoxia at the tip of the villus is intensified, leading to the death of enterocytes at the villus tip.

Question 17

Functional Implications of the Countercurrent Exchange potential problem

Answer 17

i. Oxygen delivery impeded: O2 is high in arterial blood but low in venous blood. Therefore O2 diffuses from artery to vein, so arterial blood O2 content falls as blood approaches villus tip. Countercurrent loss of O2 is thought to be a factor contributing to the high sensitivity of villus cells to ischemic hypoxia. Prolonged ischemia will lead to cell death, sloughing of mucosa and if severe will produce perforation of bowel wall, shock and death.
ii. Na+ is concentrated in the paracellular space at the villus tip. Na+ is absorbed by enterocytes from the base to the tip of a villus. Since flow of blood originates at the villus base, and venules return blood centrally within the center of the villus, paracellular osmolarity become very high at the villus tip. Consequently, arterial osmolarity also is increased as blood approaches villus tip. The net result is an increase in villus tip osmolarity (600-1200 mosm) which attracts H2O from the lumen during periods of high Na+ absorption. Essentially this process sets up a strong localized driving force for water absorption into the villus capillary network.

Question 18

Bacteria make up more than […] of stool by weight.

Answer 18

Bacteria make up more than 30% of stool by weight.

• However, chyme in the jejunum contains few bacteria. In fact, nearly all bacteria in the alimentary canal reside in the colon under normal conditions. The high acidity of the stomach and proximal duodenum retard bacterial entrance and growth, whereas the jejunum and ileum provide optimal proliferation conditions.
• It is clear that a competent ileal-cecal sphincter is important for keeping bacteria in the colon. In addition, as indicated above, motility and constant flushing of intestinal contents plays a role as well.
• A major factor in the high level of bacterial growth in the colon is the relatively low peristaltic activity. Significant infiltration of bacteria into the small intestine is often associated with maldigestion and malabsorptive problems.

Question 19

Protective Mechanism of the Alimentary Canal

Answer 19

A. HCl in Stomach: Kills most bacterial entering in food

B. Ileal-Cecal Sphincter (Pyloric, LES): Maintain unidirectional flow and thereby control over movement of bacteria and other noxious agents

C. Migrating Motor Complex: Maintains bacteria in colon and removes material from stomach and small intestine during inter-digestive periods.

D. Intact Epithelial Lining and Portal System: regulate movement of bacterial and agents between the lumen and systemic blood.

E. Gut Associated Lymphoid Tissue (GALT), dendritic cells and Liver Kuppfer Cells

Question 20

Mechanisms that Protect Against Systemic Bacterial Infection

Answer 20

•The mechanisms by which bacterial over-growth is controlled are not well known.

However, a great deal of information has been obtained regarding the mechanisms by which the immune system protects against systemic bacterial infection from the gut.

•Within the villi of the small intestine reside an array of lymphoid cells and associated structures, which have immunologic functions.

• In the upper intestine, this amounts to solitary plasma cells and macrophages, and some small lymphoid nodules found in villi.
• In the ileum and colon are well-formed aggregates of lymphoid cells called “Peyers Patches”. The combination of cells and structures is referred to as the “gut associated lymphoid tissue” or GALT.
• In addition, it is now believed that inflammatory responses initiated by the GALT system are important for stimulating neural reflexes that coordinate elevations of epithelia secretion in response to infectious insults. “Selective excitatory and inhibitory reflexes coupled to direct actions of inflammatory mediators result in motility and secretory patterns characterized by strong contractions, copious secretion and diarrhea”. This in effect promotes flushing of the colon and purging of the offending toxin- microbe

Question 21

Peyer’s Patches

Answer 21

• In the ileum and colon are well-formed aggregates of lymphoid cells called “Peyers Patches”.
• Peyers patches are considered initiating sites for mucosal immunity since they harbor B-lymphocytes. These cells produce and secrete primary immunoglobulins.

Question 22

In addition to the GALT system, macrophages resident in the liver parenchyma referred to as […] play an important role as a defensive mechanism against bacteria that enter the portal system.

Answer 22

In addition to the GALT system, macrophages resident in the liver parenchyma referred to as Kuppfer cells play an important role as a defensive mechanism against bacteria that enter the portal system.

Question 23

Dendritic Cells

Answer 23

• The mucosal immune system constantly surveys the intestinal microbiota. The outcome of this interaction is determined by dendritic cells that facilitate antigen sampling and pathogen recognition.
• Dendritic cells populate the entire lamina propria of the gastrointestinal tract as an extensive network.
• Distinct dendritic cell subsets may be associated with specific immune functions in the lamina propria and Peyer’s patches.
• These cells form transepithelial dendrites that allow dendritic cells direct access to the intestinal lumen to obtain information about commensal and pathogenic microorganisms.
• This information is then presented to their basolateral membrane to regulate the function of the intestinal immune system by directing intestinespecific migration and control of T cells.